Polymer-layered nanocomposites often exhibit physical and chemical properties that are dramatically different from conventional microcomposites. These polymer layered nanocomposites can exhibit increased modulus, decreased thermal expansion coefficients, reduced gas permeability, increased solvent resistance and enhanced ionic conductivity when compared to the polymers alone.
In general, two idealized polymer-layered nanocomposite structures are possible: intercalated and exfoliated. The greatest property enhancements are generally observed for exfoliated nanocomposites. These consist of individual nanometer-filler layers suspended in a polymer matrix. In contrast, intercalated hybrids consist of well ordered multilayers with alternating polymer/nanometer-filler layers with a repeat distance of a few nanometers. In reality many systems fall short of the idealized exfoliated morphology. More commonly, partially exfoliated nanocomposites, containing small stacks of 2-4 nanometer-filler layers uniformly dispersed in the polymer medium, are obtained. Nevertheless, these systems may still exhibit substantial physical property enhancements.
Intercalated and exfoliated poly(methyl methacrylate) (PMMA) nanocomposites including nanometer-thick silicate layers dispersed throughout the polymer matrix have been studied in the prior art. PMMA is an important member in the family of polyacrylic and methacrylic esters. PMMA has several desirable properties, including exceptional optical clarity, good weatherability, high strength and excellent dimensional stability. PMMA nanocomposites offer the potential for reduced gas permeability, improved physical performance, and increased heat resistance, without a sacrifice in optical clarity.
The literature contains several reports on the interaction of PMMA and layered silicates. Biasci and co-workers, F. Polymer 1994, 35, 3296 obtained intercalated PMMA/clay hybrid structures by two methods: (1) polymerization of MMA with montmorillonite modified by 2-(N-methyl-N,N-diethylanunonium iodide)ethyl acrylate, and (2) direct interaction of MMA copolymers (which contained pendent ammonium groups) with an organically modified silicate. Chen and co-workers, J. Mater. Sci. Lett., 1999, 18, 1761, and Okamoto and co-workers, Polymers 2000, 41, 3887, prepared partially exfoliated PMMA nanocomposites by the bulk polymerization of MMA in the presence of an organically modified silicate. Dietsche and co-workers, PMSE Preprints 2000, 82, 222, prepared an amidinium-functionalized PMMA (Mn=3200 g/mol) and used this material as an organic modifier for fluorohectorite. Effective exfoliation was observed using this oligomeric modifier in the bulk polymerization of MMA and MMA/comonomer mixtures. This result is consistent with the view that tethered polymer chains promote exfoliation.
At least two reports describe the preparation of PMMA/clay hybrids by emulsion polymerization of MMA. In both reports, the layered silicate is present during polymerization. Lee and Jang, J.Appl. Polym. Sci. 1996, 61, 1117, only observed intercalated structures. Recent work by Bandyopadhyay, Giannelis and Hsieh demonstrated formation of an exfoliated PMMA nanocomposite; a 6° C. increase in Tg and a 50° C. increase in the decomposition temperature was reported for the PMMA nanocomposite.
Although considerable research has focused on providing PMMA nanocomposites, the emulsion polymerization process herein disclosed employs an experimental method heretofore unknown in that the nanocomposite is formed in a post polymerization step. Additionally, although the discussion above focused considerably on PMMA nanocomposites, the present invention will not be limited thereto. Rather, the present invention focuses on a method for creating a broad scope of polymer-layered nanocomposites, and may encompass the use of a multitude of different polymers and a multitude of different nanofillers. However, PMMA nanocomposites are provided in one preferred embodiment.